Internal combustion engines include exhaust control mechanisms for controlling the flow of exhaust from each combustion chamber of the engine. In most instances, these mechanisms comprise valves.
In those instances where the engine operates on a two-stroke or two-cycle principal, the valve is often of the sliding or rotating type. Generally, this type of valve does not serve to ever completely close the opening or port in the combustion chamber wall or the exhaust passage. Instead, the valve moves between a first position in which the valve does not obstruct or obstructs very little of the exhaust port, and a second position in which the valve partly obstructs the port. In the first position, the timing of the opening of the exhaust port as the piston moves down is advanced, and the timing of the closing of the exhaust port as the piston moves up is delayed, providing for a longer exhaust duration as compared to that when the exhaust timing valve is in the second position. By moving the exhaust timing valve between its first and second positions or positions therebetween, the timing and duration of the exhaust flow, and thus the performance or running conditions of the engine, can be adjusted.
A problem arises when an engine having this type of valve is used to power a planing type boat. This type of boat is arranged so that below a certain speed, the hull displaces a great deal of water as it moves through the water. Above a certain speed, however, the boat rises out of the water into a "planed" condition. In this planed condition, less of the hull is positioned in the water.
As may be appreciated, when the boat moves from the non-planed to the planed condition, the hull resistance decreases substantially, lowering the load on the motor. Of course, as the speed of the boat continues to increase in the planed condition, the hull resistance rises and at some point may actually be higher than that before the boat moved to its planed condition.
The exhaust timing valve of this type of engine is conventionally operated in the following manner. When the boat is being operated at speed below a predetermined high speed (this speed normally being higher than the speed at which the boat planes), the exhaust valve is moved to its substantially closed position. In this position, the compression ratio and combustion time are increased, providing high engine power and torque.
When the boat speed increases above the predetermined high speed, the engine speed must similarly be increased. At this time, the exhaust valve is moved to its open position. In this position the exhaust gases flow more freely from the combustion chamber, lowering exhaust resistance and permitting higher engine speeds.
The problem arises that this exhaust control strategy presents problems when the operator of the boat wishes to accelerate the boat quickly from the non-planing state to the planing state. When the exhaust timing valve is closed, quick acceleration of the engine is inhibited by the exhaust gas resistance.
An engine control for an engine powering a planing-type boat which overcomes the above-stated problems is desired.